In their attempt to lose weight, obese individuals frequently give a history of repeated cycles of gain and loss of weight. The question is addressed in this proposal whether cycles of weight gain and loss might lead to greater ease of weight gain and increase adiposity with increased adipose cellularity. We will attempt to prove this hypothesis and investigate its mechanisms using the Osborne-Mendel rat which is easily induced to gain weight with a high-fat diet and lose weight with a modified fast. Comparisons will be made between weight cycled obese, continually obese and normal weight rats for body composition, energy content, feed efficiency, adipocyte size and number, hormones (insulin and thyroid hormones) and several enzymes from adipose and liver tissue.